its illegal according to US law. and as per the rules of 4chan, it isnt allowed here as well. /diy/ isnt here to help you with illegal activities.
Actually EMP jammers don't interfere with wireless communications.
To do that you'd have to constantly overpower a specific radio frequency.
At best you could say that an EMP jammer would jam a wireless device if it were close enough, but you can't actually make a large enough EMP jammer that it wouldn't be a toy.
The biggest ones I've seen are made from microwave magnetrons.
What is it? Does it generate a single EMP to permanently damage electronics, continously generate discrete frequencies to jam wifi or cellphones, or is it a device to induce electromagnetic interference to mess things up?